Weight gain and weight loss are about calories in and calories out. It's not genetics, it's not anything else. I recommend actually counting all of your calories, carbs, sugars, and proteins for a single, average day, just for some perspective.
I also recommend eating less, as in as close to zero sugars, more fiber, and a lot more protein.
Weight loss is 90%diet, 10%exercise. Pay attention to exactly what you're eating.